Electron gun device of field emission type

ABSTRACT

In an electron gun device of the field emission type, a shield electrode whose potential is the same as that of a cathode-tip of needle shape is disposed in surrounding relationship to a first anode so as to shield said first anode from ground potential portions of the device, such as the envelope, so that discharge between the anode and the ground potential portions is prevented, whereby the needle tip of the cathode is protected from destruction due to said discharge.

o United States Patent 1191 11 1 mamas Komoda 1 1 May 1, 1973 541ELECTRON GUN DEVICE 0F FIELD 3,283,203 11 [1966 Dyke et al. ..313/240EMISSION TYPE 3,132,275 5/1964 Merdinian ..313/82 R [75] Inventor:Tsutomu Komoda, Kodaira-shi, OTHER PUBLICATIONS T k J 0 yo apanElectronmrcroscopy by Igor Bensen, Genera] Elec- Assigneer Hitachi, yJapan tric Review, December 1944, pp. 6-12.

22 i=1 a; A .1 1971 a 1 l 8 pr Primary Examiner-James W. Lawrence [21]Appl. No.: 130,283 Assistant ExaminerHarold A. Dixon Attorney-Craig andAntonelli [52] U.S. Cl. ..250/49.5 A, 250/495 R, 313/82 51 1m. 01 .110137/26 [57] ABSTRACT [58] Field of Search ..3l3/82, 83, 206, In anelectron gun device of the field emission type, a 0; 250/ 95 R shieldelectrode whose potential is the same as that of a cathode-tip of needleshape is disposed in surround- [56] Referentes C ted ing relationship toa first anode so as to shield said first anode from ground potentialportions of the UNITED STATES PATENTS device, such as the envelope, sothat discharge 2,536,878 1 1951 Fleming ..2s0 49.s c between the anodeand the ground potential portions 2,289,071 7/1942 Ramo is prevented,whereby the needle tip of the cathode is 2,245,998 6/1941 Pietschprotected from destruction due to said discharge. 3,436,583 4/1969Hughes 2,928,943 3/1960 Bartz et al. ..250/49.5 A 10 Claims, 2 DrawingFigures B 151* ANODE CATHODE WI I 2 SHIELD 5 2 2 4 /Z x w f/ 6.

2nd ANODE ELECTRON GUN DEVICE'GF FIELD EMISSION TYPE BACKGROUND OF THEINVENTION As is well known, a conventional electron gun device l for anelectron microscope has a cathode tip of needle shape, means for heatingsaid tip and an anode for causing electrons to be emitted from aid tipby applying an electric field between said tip and said anode.

In such an electron gun device, undesirable discharge occurs frequentlyin the operation and at this time, the tip portion of said cathode isdestroyed so that the field emission of electrons therefrom stops.

Since the useful life of the cathode tip in the conventional electrongun device is very short for the reasons described above, it isnecessary to exchange said tip frequently.

In the electron gun device, ultra high vacuum conditions must bemaintained and once this condition is broken for exchange of said tip,it is necessary to evacuate the electron gun device once again. However,this requires complicated procedures and very long time to effect suchevacuation.

SUMMARY OF THE INVENTION An object of the invention is to provide anelectron gun device havinga very long useful life.

Another object of the invention is to provide an electron gun devicehaving the advantage that the number of times wherein exchange of thecathode tip is required, and thus the number of times wherein evacuationof the microscope is necessary, has been greatly reduced. v

A further object of the invention is to provide an electron gun devicesuitable as the source, of an electron beam for use in an electronmicroscope and the like.

The electron gun device of this invention is characterized in that ashield electrode, being at the same voltage as that of the anode, isdisposed in said device to surround an accelerating anode for shieldingsaid anode from parts at ground potential.

These and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription when taken in conjunction with the accompanying drawings,wherein:

FIG. 1 is a schematic diagram showing an embodiment of the invention;and

FIG. 2 is a schematic diagram showing another embodiment of theinvention.

DESCRIPTIQN OF THE PREFERRED EMBODIMENTS In FIG. I, an electron gun foran electron microscope includes a cathode tip 1 of needle shape, a firstanode 2, asecond anode 3, the respective anodes being provided withholes 4 and 5 through which an electron beam passes. Insulators 6 and 11serve as supports and spacers, the insulator 6 carrying terminalelectrodes 7 supporting a tungsten filament 8 of hair-pin shape, and aterminal electrode 9 carrying a contact spring 10.

These are well known elements which constitute a conventional electrongun structure. However, in addition, a first shield electrode 12 isprovided which is a characteristic element according to the presentinvention.

In a conventional electron gun structure, a negative high voltage, forexample 30 KV, is applied to the cathode tip 1 through the terminalelectrodes 7, and another negative high voltage of about -27 RV is ap- 0plied to the first anode through the contact spring i0 and the terminalelectrode 9. Moreover, the second anode 3 is grounded.

In the electron gun device of conventional structure, since theintensity of the electric field adjacent to the top portion of thecathode tip 1 becomes 10 -10 v/cm due to potential difference betweensaid tip 1 and the first anode 2, electrons are caused to emit from saidtip 1 by means of the electric current flowing through the filament 8 soas to make it clean by heating it. These electrons passing through hole4 are focused by an electric field produced between the first and secondanodes 2 and 3 which act as an-electron lens so that an electron beamhaving a fine spot of high density is obtained from the hole 5.

Since the cathode tip 1 is surrounded by the first anode 2, dischargealmost never occurs between said tip 1 and parts which are not at groundpotential, such as the envelope of the electron gun device (not shown),even when the negative high voltage is applied to the cathode tip 1 asdescribed above.

However, it is possible that discharge occurs between the first anode 2and the parts at ground potential. When such discharge occurs, thepotential of the first anode 2 is reduced to ground potential at once,and the potential ditterence between the tip 1 and the first anode 2becomes very high so that the quantity of electrons emitted from the tip1 increases rapidly and to a great extent.

Generally, the tip portion of the cathode l is very sharp so as to allowelectrons to be emitted therefrom by means of electric field effects.For example, a tungsten line, having a diameter of 0.1-0.2 mm whoseradius of curvature at the tip portion is formed in a nee- I dle shapeto have the very small value of 500 A, is used as the tip. Therefore, asa result of the rapid increase of electrons upon occurrence of theabove-mentioned discharge, the tip portion of the cathode isextraordinarily heated and thereby destroyed, so that emission ofelectrons therefrom stops since the intensity of the electric fieldadjacent to the tip portionof the cathode 1 decreases to almost zerovalue, under which conditions the field is unable to cause electrons tobe emitted therefrom.

It is apparent from the foregoing description that the positivepotential difference between the cathode tip and the first anode can beprevented from becoming excessive by preventing discharges between thefirst anode and the ground potential parts of the electron gun assemblyand thereby destruction of the tip portion of the cathode can beprevented.

Thus, the electron gun device of the present invention is designed tohave a first shield electrode 12 sur- 7 rounding the first anode 2. Thesame voltage that is applied to the tip 1 is applied to the first shieldelectrode 12 through the contact spring l3 connec ted to the terminalelectrode 7.

Moreover, an insulator capable of withstanding voltage of at least 5 KVis disposed between the first anode 2 and the first shield electrode 12,and if necessary, a second shield electrode 14 may be disposed tosurround the end portion 6' of the insulator 6 so as to prevent adischarge therefrom.

The first shield electrode 12 has a bottom portion 16 which shields thefirst anode 2 from the second anode 3 as well as from said groundpotential portions, but since the bottom portion 16 of the shieldelectrode 12 has a curved smooth surface which is fully polished so asnot to cause any large variation in the distribution of an electricfield between said anodes, almost no discharge may occur therebetween.

On the contrary, discharge may occur easily between contact portions 17and 18, or between contact portions 19 and 20 along the insulator 11between the two anodes. Therefore, the first shield electrode can bemodified to practically omit the bottom portion 16 and to dispose in itsplace a projected portion 12.

According to this invention, since the top portion of the tip isprotected from destruction due to said discharge, the life time of thetip greatly increases. This invention can be applied to all electronicgun devices of the field emission type having at least a cathode tip ofneedle shape and an anode for producing the electric field between itand the tip so as to cause electrons to be emitted from the tip.

While I have shown and described two embodiments in accordance with thepresent invention, it is understood that the same is not limited theretobut is susceptible of numerous changes and modifications as known to aperson skilled in the art, and I therefore do not wish to be limited tothe details shown and described herein but intend to cover all suchchanges and modifications as are obvious to one of ordinary skill in theart.

What is claimed is:

1. In an electron gun device of the field emission type for generatingan electron beam having a cathode tip of needle shape, means for heatingsaid cathode tip, a first anode, a second anode, and means for applyingan electric voltage between said cathode tip and said first anode so asto produce an electric field therebetween which causes electrons to beemitted from said tip, the improvement which comprises a shieldelectrode disposed to surround said first anode between said first andsecond anode and means for applying to said shield electrode the samevoltage as that applied to said anode, whereby said shield electrodeshields said anode from ground potential portions of said device.

2. An electron gun device as defined in claim 1, wherein said anode andsaid shield electrode are substantially concentric cup-shaped elements.

3. An electron gun device as defined in claim 1, wherein said means forheating said cathode tip includes a pair of terminals connected to saidcathode tip for applying a heating current thereto and an insulator forsupporting said pair of terminals, and further including an additionalshield electrode disposed between said insulator and said anode andbeing connected to one of said pair of terminals.

4. An electron gun device as defined in claim 3, wherein said means forapplying a voltage to said shield electrode includes an electricalconnection between said shield electrode and said additional shieldelectrode.

5. An electron gun device as defined in claim 1, wherein said shieldelectrode is a substantially cylindrical element disposed concentricallyto said anode.

6. An electron gun device of the field emission type comprising:

a cathode tip of needle shape capable of being heated and having a largenegative electric potential with respect to ground potential portions ofsaid gun device;

an anode applied with an accelerating voltage with respect to said tipso as to produce therebetween an electric field which causes fieldemission of electrons from said tip; and

a shield electrode disposed to surround said anode and interposedbetween said anode and said ground potential portions of the gun device,said shield electrode being rendered in the same potential as that ofsaid tip, whereby said shield electrode shields said anode from theground potential portions to thereby prevent undesired dischargetherebetween.

7. An electron gun device as defined in claim 6, wherein said anode andsaid shield electrode are substantially concentric cup-shaped elements.

8. An electron gun device as defined in claim 6, wherein said shieldelectrode is a substantially cylindrical element disposed concentricallyto said anode.

9. An electronic gun device as defined in claim 6, further includingmeans for heating said cathode tip including a pair of terminalsconnected to said cathode tip for applying a heating current thereto andan insulator for supporting said pair of terminals, and an additionalshield electrode disposed between said insulator and said anode andbeing connected to one of said pair of terminals.

10. An electron gun device as defined in claim 9, wherein said means forapplying a voltage to said shield electrode includes an electricalconnection between said shield electrode and said additional shieldelectrode.

1. In an electron gun device of the field emission type for generatingan electron beam having a cathode tip of needle shape, means for heatingsaid cathode tip, a first anode, a second anode, and means for applyingan electric voltage between said cathode tip and said first anode so asto produce an electric field therebetween which causes electrons to beemitted from said tip, the improvement which comprises a shieldelectrode disposed to surround said first anode between said first andsecond anode and means for applying to said shield electrode the samevoltage as that applied to said anode, whereby said shield electrodeshields said anode from ground potential portions of said device.
 2. Anelectron gun device as defined in claim 1, wherein said anode and saidshield electrode are substantially concentric cup-shaped elements.
 3. Anelectron gun device as defined in claim 1, wherein said means forheating said cathode tip includes a pair of terminals connected to saidcathode tip for applying a heating current thereto and an insulator forsupporting said pair of terminals, and further including an additionalshield electrode disposed between said insulator and said anode andbeing connected to one of said pair of terminals.
 4. An electron gundevice as defined in claim 3, wherein said means for applying a voltageto said shield electrode includes an electrical connection between saidshield electrode and said additional shield electrode.
 5. An electrongun device as defined in claim 1, wherein said shield electrode is asubstantially cylindrical element disposed concentrically to said anode.6. An electron gun device of the field emission type comprising: acathode tip of needle shape capable of being heated and having a largenegative electric potential with respect to ground potential portions ofsaid gun device; an anode applied with an accelerating voltage withrespect to said tip so as to produce therebetween an electric fieldwhich causes field emission of electrons from said tip; and a shieldelectrode disposed to surround said anode and interposed between saidanode and said ground potential portions of the gun device, said shieldelectrode being rendered in the same potential as that of said tip,whereby said shield electrode shields said anode from the groundpotential portions to thereby prevent undesired discharge therebetween.7. An electron gun device as defined in claim 6, wherein said anode andsaid shield electrode are substantially concentric cup-shaped elements.8. An electron gun device as defined in claiM 6, wherein said shieldelectrode is a substantially cylindrical element disposed concentricallyto said anode.
 9. An electronic gun device as defined in claim 6,further including means for heating said cathode tip including a pair ofterminals connected to said cathode tip for applying a heating currentthereto and an insulator for supporting said pair of terminals, and anadditional shield electrode disposed between said insulator and saidanode and being connected to one of said pair of terminals.
 10. Anelectron gun device as defined in claim 9, wherein said means forapplying a voltage to said shield electrode includes an electricalconnection between said shield electrode and said additional shieldelectrode.